This invention relates to measurements of electrical voltages and in particular to measurement of rms values of time-varying voltages.
When measuring the voltage of an electric signal, it is useful to represent the voltage value by a single number, even though the voltage may be varying rapidly in time. One common measurement is the xe2x80x9cpeakxe2x80x9d voltage, which represents the maximum magnitude present in the signal. In a sinusoidal signal, for example, the peak voltage is one half the voltage difference between a minimum and a maximum of the sine curve.
It is often more useful to represent a time-varying voltage by some type of average value that would correspond to an equivalent direct current (DC) voltage, because the equivalent DC current determines the energy loss or heating caused by applying a voltage across a resistor. A simple arithmetic average of the voltage over time is typically not useful because time varying signals, such as an alternating current (AC) signal in which the voltage varies sinusoidally between positive and negative values, often have an average voltage over time of approximately zero. A more useful value to represent the time varying voltage is the root mean square (xe2x80x9crmsxe2x80x9d) value, which is the square root of the integral of the square of the voltage over time. The ratio of the peak voltage to the rms voltage of a signal is known as the xe2x80x9ccrest factor.xe2x80x9d
Systems for determining an rms voltage from a time-varying incoming signal are known as rms converters, a common type of which is the log-antilog rms converter described in U.S. Pat. No. 4,389,708, which is assigned to the assignee of the present invention.
Electrical measurement instruments are typically accurate over a limited range, in part because circuit components in the instruments are linear over only a limited range. Thus, measuring devices have difficulty measuring signals that have a high crest factor, that is, signals that include peak voltage values that are significantly larger than the rms value. Inaccurate measurement of the high peak voltages adversely affects the accuracy of the calculated rms value.
The ratio of the highest peak that can be accurately measured to the maximum rms value is called the crest factor limitation of the measuring device. Known methods for increasing the crest factor limitation by shrinking the incoming signal to fit the device capability do so by sacrificing measurement accuracy at lower voltages. The signal being measured, however, typically has a low voltage over most of the measurement interval and low voltages, therefore, contribute the most to the rms calculation.
Analog gain correction methods are known for extending the crest factor limitation, but such methods require switching the gain at the front end of the analog rms converter. This gain switching creates problems with settling time, overshooting, and accuracy at the lower voltages. Extending both range and accuracy of a measuring device requires more complex and costly components and circuits.
Another method is to use two rms converters, one to process high incoming voltages and one to process receive low voltages. The outputs of the two rms converters are then combined in an analog adder with appropriate scaling. The use of two rms converters increases expense and size of the resultant device.
The crest factor limitation problem is particularly acute in rms converters that convert the incoming signal to digital values before determining the rms value. The additional capacity required for digitally processing a high peak and converting it from an analog signal into a digital value is costly, and is seldom used, because the majority of the signal is well below the maximum anticipated peak.
In accordance with the invention, a high crest factor, time-varying signal can be easily and accurately converted to an rms value.
Accordingly, it is an object of the present invention to provide an improved method and apparatus for determining an rms value of an incoming signal.
It is a further object of the present invention to provide such an improved method and apparatus that can accurately measure incoming signals having high crest factors.
It is yet another object of the present invention to provide such a method and apparatus that can utilize components having limited dynamic range.
The present invention is a method and apparatus for processing an incoming signal at the front end of an rms converter and a method and apparatus for determining an rms value of a time-varying electrical signal. In accordance with the present invention, an incoming, time-varying signal is processed by an amplifier having a transfer function of non-uniform slope. Higher voltage portions of the incoming signal are amplified less than lower voltage portions, thereby reducing the maximum voltage of the signal output from the amplifier.
When the output signal is processed to determine an rms value, the processing includes compensating for the known non-uniform slope of the transfer function. By using a transfer function of non-uniform slope and compensating for the non-uniformity in the processing step, the crest factor of the measuring device is increased without the necessity of increasing the dynamic range of the components following the amplifier.
In a preferred embodiment, the transfer function comprises a line having a change in slope at a voltage of a predetermined magnitude. The slope of the line decreases above the predetermined magnitude, thereby reducing the amplifier output voltage value for large signals. The amplifier output is fed into an analog to digital converter and is then digitally processed to produce an rms value. The digital processing includes weighting the signals in accordance with the appropriate gain factor, so that each part of the time-varying signal contributes appropriately to the rms calculation.
The invention thus accommodates the infrequent peaks of high crest factor signals while still maintaining accuracy at the lower voltages levels of the preponderance of the incoming signal. The simplicity and low cost solution to high crest factor rms measurement provided by the present invention makes it particularly suitable for application in hand-held or small bench top multimeters.
The subject matter of the present invention is particularly pointed out and distinctly claimed in the concluding portion of this specification. However, both the organization and method of operation, together with further advantages and objects thereof, may best be understood by reference to the following description taken in connection with accompanying drawings wherein like reference characters refer to like elements.